Validation of OMI NO2 data using ground-based spectrometric NO2 measurements

at Zvenigorod, Russia

A.N. Gruzdev and A.S. Elokhov

A.M. Obukhov Institute of Atmospheric Physics Moscow

Zvenigorod (55.7°N, 36.8°E)

• The station is located in a rural area 50 km west of Moscow.

• The station is exposed to pollution episodes most frequent and intensive in winter

Zvenigorod

Kislovodsk

Lovozero

Minsk

Issyk-Kul

Tomsk

Petrodvorets

ZhiganskSalekhard

The method of NO2 measurements• Measurements are done by a zenith viewing grating scanning monochromator MDR-23 in 435-450 nm wavelength spectral range (versus 415-465 nm for OMI)• Spectral resolution is 0.7 nm (versus ~0.5 nm for OMI)•Measurements are done in morning and evening twilight at solar zenith angles 84-96°•The instrument and method of measurements of a slant column NO2 were certified by the NDACC following the results of intercomparison carried out at Zvenigorod in 1997• Using measured slant column NO2 abundances, vertical distribution of NO2 is retrieved by solving inverse problem with Chahine method• A NO2 vertical profile is represented by NO2 abundances in ten 5-km thickness layers and in the thin surface layer• The method allows separating unpolluted and potentially polluted parts of the NO2 column content

Examples of NO2 vertical profiles at Zvenigorod retrieved for conditions of clean and polluted boundary layer

During pollution episodes, the NO2 abundance within the atmospheric boundary layer can be significantly larger than in the above troposphere-stratosphere layer

The profiles correspond to solar zenith angle 84°

0 0.5 1 1.5 2 30 40N O 2 layer content (1015 cm -2)

0

10

20

30

40

50

Alti

tude

(km

)

18 June, 2007, evening23 June, 2007, m orn ing

The axis is broken

Comparison with data of SAGE II and CRISTA 2 satellite measurements

CRISTA 2 август 1997 г.

SAGE II

Alti

tude

(km

)

Alti

tud

e

(km

)A

ltitu

de

(km

)

NO2 concentration (108 mol/cm3)

NO2 concentration (108 mol/cm3)

7 April 1993

23 March 1992

Zvenigorod

Zvenigorod

SAGE II (52N, 39.2E)

SAGE II (57.7N, 41.5E)

August 1997

ZvenigorodCRISTA 2

Methodology of comparison with data of OMI measurements

• In deriving slant column NO2 abundances, the NO2 absorption cross sections by Vandaele et al. (1998) are used corresponding to temperature 220 K• The distance of OMI ground pixel centers from the Zvenigorod station is chosen to be less than 45 km• NO2 contents retrieved from ground-based measurements correspond to solar zenith angle 84° • Data of ground-based measurements are then interpolated to time of OMI overpass measurements with the help of photochemical modeling

0 3 6 9 12 15 18 21 24Loca l tim e (hour)

0.4

0.8

1.2

1.6

Str

atos

phe

ric c

olum

n N

O2

(

rela

tive

units

)

D ecem ber 22M arch 22June 22Septem ber 22

Stratospheric co lum n N O 2 d iurna l varia tion

Quantities used for comparison

OMI Ground-based

1 “Unpolluted” vertical column NO2 abundance

(NO2Unpol product)

Vertical column NO2 abundance above the surface layer (includes NO2 contents in ten 5-km thickness layers)

2 Tropospheric vertical column NO2 abundance

(NO2Trop product)

Tropospheric (0-10 km) vertical column NO2 abundance (includes NO2 contents in two tropospheric 5-km thickness layers and in the surface layer)

Comparison of OMI and ground-based measurements

“Unpolluted” column NO2

2005 2006 2007 2008Year

0

1

2

3

4

5

6

NO

2 c

onte

nt (

101

5 c

m-2

)

O M Iground-based, m orn ingground-based, evening

OMI data are generally between morning and evening ground-based data

“Unpolluted” column NO2

2005 2006 2007 2008Year

0

1

2

3

4

5

6N

O2 c

onte

nt (

101

5 c

m-2

)O M IG round-based, in terpolated

Difference (OMI - ground-based)

Mean difference: –(0.300.03)∙1015 cm-2

or –(11.2 1.2)%

Mean square root diff.:

0.6∙1015 cm-2 or 22%

Mean difference:–(0.170.04)∙1015 cm-2

or –(6.9 1.6)%

Mean square root diff.: 0.47∙1015 cm-2 or 19%

-3

-2

-1

0

1

2

Diff

eren

ce,

OM

I-G

B (

101

5 с

м2)

For a ll data

2005 2006 2007 2008Year

-3

-2

-1

0

1

2

For O M I tropospheric co lum n N O 2 < 2 .10 15 cm -2

Correlation of “unpolluted” column NO2 values

Correlation coefficient ~0.9

Linear regression equation

OMI = 0.82GB + 0.2∙1015 cm-2

0 1 2 3 4 5G round-based unpolluted colum n NO 2 (10 1 5 cm -2)

0

1

2

3

4

5

OM

I un

pollu

ted

colu

mn

NO

2 (

101

5 c

m-2

)

Possible reasons of discrepancies between OMI and ground-based stratospheric NO2 data

• Different spectral regions used for measurements• Different spectral resolutions of the two instruments• Different methods of measurements resulting, in

particular, in different spatial resolution (smoothing) of results of measurements

• Different sensitivity to tropospheric pollution

• Temperature dependence of NO2 cross sections that may differently affect derived NO2 contents due to different spectral regions and spectral resolutions of the two instruments

Comparison of OMI and ground-based measurementsTropospheric column NO2

• There is a significant discrepancy between OMI and ground-based estimates of tropospheric NO2 contents• It is related to strong spatial inhomogeneity ant temporal variability of pollution part of NO2 and is due to different spatial averaging of satellite and ground-based observations

2005 2006 2007 2008

0

20

40

60

80

100

120

140

NO

2 c

onte

nt (

101

5 c

m-2

)

O M Iground-based, m orn ingground-based, evening

2005 2006 2007 2008Year

0

20

40

60

80

100

120

140

NO

2 c

onte

nt (

101

5 c

m-2

)

g round-based, in terpola ted

Mean OMI-GB difference: –(1.40.5)∙1015 cm-2

Mean square root difference7.9∙1015 cm-2 (~200%)

Correlation of tropospheric column NO2 values

Correlation coefficient for daily data

~0.4

Linear regression equation

OMI = 0.22GB + 2.2∙1015 cm-2

Correlation coefficient for monthly mean data

~0.45

Linear regression equation

OMI = 0.16GB + 2.3∙1015 cm-2 0 10 20 30 40 50 60 70

G round-based tropospheric co lum n NO 2 (10 1 5 cm -2)

0

10

20

30

40

50

OM

I tr

opos

pher

ic c

olum

n N

O2 (

101

5 c

m-2

) da ilym onth ly m ean

Conclusions

• OMI “unpolluted” NO2 columns underestimate ground-based measurements at Zvenigorod by (0.300.03)∙1015 molecules/cm-2 (~11%), if all data are used, and by (0.170.04)∙1015 molecules/cm-2 (~7%), if a part of data are used for comparison for which OMI tropospheric NO2 columns is not large (< 2∙1015 cm-2)

• OMI tropospheric NO2 columns are on average by (1.40.5)∙1015 molecules/cm-2 (~35%) less than those derived from ground-based measurements

• On the whole, more detail investigation of lower tropospheric pollution effects is needed for more accurate validation of OMI data